A fuel cell using a polymer electrolyte membrane generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air. This fuel cell is basically composed of a polymer electrolyte membrane for selectively transporting hydrogen ions; and a pair of electrodes, i.e., an anode and a cathode, formed on both surfaces of the polymer electrolyte membrane. The electrode usually comprises a catalyst layer which is composed mainly of a carbon powder carrying a platinum group metal catalyst and formed on the surface of the polymer electrolyte membrane; and a diffusion layer which has both gas permeability and electronic conductivity and is formed on the outer surface of the catalyst layer.
In order to prevent the fuel gas and oxidant gas supplied to the electrodes from leaking out or prevent these two kinds of gases from mixing together, gaskets are arranged on the periphery of the electrodes with the polymer electrolyte membrane therebetween. The gaskets are combined integrally with the electrodes and the polymer electrolyte membrane beforehand. This is called “MEA” (electrolyte membrane-electrode assembly). Disposed outside the MEA are conductive separator plates for mechanically securing the MEA and for connecting adjacent MEAs electrically in series, or in some cases, in parallel. The separator plates have a gas flow channel for supplying a reaction gas to the electrode surface and for removing a generated gas and an excess gas, in a portion that comes into contact with the MEA. Although the gas flow channel may be provided separately from the separator plates, grooves are usually formed on the surfaces of the separator plates to serve as the gas flow channel. Also, a method in which the gas flow channel grooves are formed on the electrodes has been proposed, depending on the circumstances.
In order to supply the fuel gas and oxidant gas to these grooves, it is necessary to use piping jigs which branch respective supply pipes for fuel gas and oxidant gas, according to the number of separator plates to be used, and connect the branches directly to the grooves of the separator plates. This jig is called “manifold”, and the above-described type, directly connecting the supply pipes for fuel gas and oxidant gas with the grooves, is called “external manifold”. A manifold having a simpler structure is called “internal manifold”. In the internal manifold, the separator plates with the gas flow channels formed thereon are provided with through holes which are connected to the inlet and outlet of the gas flow channel such that the fuel gas and oxidant gas are supplied directly from these holes.
Since the fuel cell generates heat during operation, it needs cooling with cooling water or the like to keep the cell under good temperature conditions. Normally, a cooling section for flowing the cooling water therein is formed every one to three cells. The cooling section is inserted between the separator plates in one structure, and the cooling section is formed by providing the backsides of the separator plates with a cooling water flow channel in the other structure. The latter structure is often employed. In a general structure of a cell stack, the MEAs, separator plates and cooling sections are alternately stacked to form a stack of 10 to 200 cells, and the resultant stack is sandwiched by end plates with current collector plates and insulating plates and is clamped with clamping bolts from both sides.
In such a polymer electrolyte fuel cell, the separator plates need to have high conductivity, high tightness against the fuel gas, and high corrosion resistance against a reaction in hydrogen/oxygen oxidation-reduction. For such reasons, conventional separator plates are made from a glassy carbon plate or a dense graphite plate, and produced by forming a gas flow channel on the surface thereof by cutting, or by placing an expanded graphite powder together with a binder in a press mold with a gas flow channel formed thereon and by heating/baking them after pressing.
In recent years, there have been attempts to use a metallic plate such as stainless steel in place of conventionally used carbon materials. In the case of the separator plate using a metallic plate, however, since the metallic plate is exposed to acidic atmosphere at high temperatures, corrosion and dissolution of the metallic plate will occur when used in a long time. If the metallic plate is corroded, the electrical resistance in the corroded portion increases, and the output of the cell decreases. Besides, if the metallic plate is dissolved, the dissolved metal ions diffuse into the polymer electrolyte and trapped at the ion exchange site of the polymer electrolyte, and consequently the ionic conductivity of the polymer electrolyte decreases. In order to prevent such deteriorations, the surface of the metal plate is normally plated with gold having a certain thickness. Furthermore, separator plates made of a conductive resin obtained by mixing a metal powder with an epoxy resin or the like have been examined (for example, Japanese Laid-Open Unexamined Patent Publication No. 6-333580).
As described above, in the conventional method of producing a separator plate by cutting a glassy carbon plate or the like, the cost of the material of glassy carbon plate is high, and, further, it is difficult to reduce the cost of cutting the glassy carbon plate. In the case of a separator plate produced by pressing expanded graphite, in order to retain the high conductivity of the separator plate, the content of the expanded graphite in the separator plate needs to be made 80 wt % or more. Accordingly, there arises a problem in the dynamic strength of the material. Therefore, the separator plate sometimes had cracks, which were caused by a deviation of the clamping load of the cell stack due to a variation in the thickness of the separator plate, more particularly vibration and impact during driving when used as the power source of an electric vehicle. If carbon fibers are added, the strength is improved, but it becomes difficult to perform injection molding as the flowability of a binder resin decreases. Furthermore, the metallic separator plates with gold plating have a problem with the cost of the gold plating. A separator plate made from a conductive resin has a lower conductivity compared to glassy carbon and metal plates, and the surface of the resin is hard. Therefore, in order to decrease the electric resistance in the portion in contact with the electrode, clamping needs to be performed at a higher pressure, and thus there is a problem that the cell structure becomes complicated.
It is an object of the present invention to provide low-cost conductive separator plates having a low volume resistivity by improving conductive separator plates composed of a binder and a conductive material consisting mainly of conductive carbon particles.
The present invention also provides a method for manufacturing such a conductive separator plate.